There are two types of defect that a flow may present which disturb the taking of measurements, and in particular measuring flow rate.
A first defect comes from asymmetry in the velocity distribution of the fluid in a plane perpendicular to the flow direction. Such asymmetry may be caused, for example, by a bend in the duct or by the shutter of a valve that is half-closed, e.g. a wedge valve.
Another defect consists in the presence of vortices that are coaxial with the duct and that may be generated spontaneously during the flow of the fluid or that may be the result of passing through a half-closed valve, as above.
In fact, neither type of defect ever exists in isolation, such that both defects are always present simultaneously in the turbulence that occurs in the fluid.
Several different devices are known that are suitable for making the velocity profile of a flow of fluid along a duct more uniform.
For example, proposals have been made to place a cylindrical chamber in the duct, the chamber including longitudinally extending ribs regularly disposed around the inside surface of the chamber. These ribs have increasing radial extent in the flow direction. Apparatuses of that type suffer from the major drawback of extending over a considerable longitudinal distance, of the order of 4 to 5 times the diameter of the inlet duct. Such a longitudinal extent is necessary to obtain a flow that has been made properly uniform. It will be understood that it increases the space required for a measurement assembly in a manner that is unfavorable.
Another device, known as a "honeycomb" comprises a large number of tubes disposed longitudinally in a cylindrical chamber installed in the duct.
In addition to the fact that such a structure is expensive both to manufacture and to install, a honeycomb apparatus requires considerable longitudinal extent, as in the preceding case.
A "star rectifier" apparatus is also known which comprises a cylindrical chamber mounted in the duct and in which walls are regularly disposed so as to constitute, in section, four or more diameters. That apparatus operates in a manner substantially similar to the honeycomb apparatus and suffers from the same drawbacks.
Those various apparatuses are discussed in the work by R. W. Miller entitled "Flow measurement engineering handbook", published by McGraw Hill, New York, 1989.
An article by K. Akaski et al., entitled "Development of a new flow rectifier for shortening upstream straight pipe length of flowmeters" published in IMEKO Tokyo, SIC, 12b-5, pp. 279-284 (1979), describes apparatus occupying much less space. The apparatus comprises a single plate pierced by constant diameter holes in a given two-dimensional distribution. Although that apparatus is compact, it nevertheless suffers from the drawback of suffering from a high headloss coefficient, of the order of two. The inevitable drop in pressure in the flow on passing through the plate gives rise to a major loss of energy.
An article by C. R. Smith et al., entitled "Using passive vortex generation devices", 5th International IMEKO Conference on Flow Measurement, Dusseldorf 1989, describes apparatus comprising two series of fins for making a flow more uniform.
Four fins are mounted on the inside face of the duct and extend parallel to its axis. They serve to transform overall rotation into four marginal vortices. No fin apparatus is envisaged for establishing vortices that are counter-rotating relative to the above vortices (thus leaving it open to the secondary marginal vortices to recombine, thereby establishing a flow that rotates in the opposite direction). The longitudinal extent of such fins is believed to be two duct diameters.
Four fins are mounted on the inside face of the duct downstream from said first series of fins, the four fins extending perpendicularly to the axis of the duct and making an angle of 30.degree. with a meridian of the duct. Each fin occupies a fraction comprising about 1/8th of the circumference of the duct and the radial extent of its projection on the axis of the duct is about 1/6th of the diameter.
A "hairpin" vortex having a longitudinal axis is established at the base of each of the fins, regardless of the overall rotation of the flow, thereby giving rise to intense turbulent mixing for the purpose of making the axial velocity distribution more uniform.
To rectify the flow, that apparatus creates a turbulent mixture that dissipates energy taken from the axial component of the velocity of the fluid, thereby giving rise to a non-negligible drop in dynamic pressure.
That device extends longitudinally over about four times the diameter of the duct, i.e. its longitudinal extent is large.
An object of the invention is therefore to rectify a flow for the purpose of making the velocity profile of a fluid flow in a duct more uniform, while occupying a distance that is very short in comparison to the diameter of said duct, thereby making it possible to reduce the bulk of a measurement assembly.
The invention also enables the velocity profile to be made more uniform without giving rise to a large headloss coefficient.